Lightning arrester



Sept. 28, 1965 D. R. PARKER LIGHTNING ARRESTER Filed Nov. 14, 1962 INVENTOR Delberr R. Parker wB j jw m ATTORNEY 5 3 5 .3 7 2 4 .l 2 8 I. l- 2 3 WITNESSES= Bm- R. (5 2M;

United States Patent 3,209,194 LIGHTNING ARRESTER Delbert R. Parker, Vienna, Ohio, assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 14, 1962, Ser. No. 237,671 12 Claims. (Cl. '31'323'1) This invention relates to improvements in lightning arresters, and more particularly to an improved expulsion type arrester in which tube material is better utilized to take up mechanical stresses in the arrester operation.

The lightning arrester of the instant invention is an improvement upon the arrester shown and described in Patent No. 2,667,072, issued Apr. 27, 1954 to E. J. DeVal for Lightning Arrester, and also upon the arrester shown and described in Patent No. 2,871,390, issued Jan. 27, 1959 to E. J. DeVal for Lightning Arrester, both assigned to the assignee of the instant invention, and has improved duty cycle performance, increased discharge current or surge current withstand ability, and an increased sustained voltage characteristic.

In these two patents, there are described improved expulsion type lightning arresters in which a filler member composed of fiber or other gas evolving material which emits a substantially un-ionized gas when the arc occurs thereacross or adjacent thereto has a helical groove cut in its surface to provide a path for the gases produced during a discharge, and which causes the gases to flow circumferentially to make erosion of the filler member uniform over its entire surface, and also to elongate the are by lengthening the path thereof, because the circumferential flow of gases has the effect of forcing the are into the helical groove, to elongate the arc and thus facilitate its interruption with a minimum of arcing time; confining the arc to the groove results in the aforementioned reduction in the amount of erosion of the insulating material, thereby increasing the life of the arrester.

As will be readily understood by those skilled in the art, two of the most important characteristics of a lightning arrester are its surge current capacity and its follow current interruption ability as measured by its duty cycle performance. The surge current capacity is a measure of the ability of the arrester to handle the exceedingly heavy current of a lightning discharge. The follow current is the momentary fiow of current from the line itself as a resultof establishing a current path through the ionized gases caused by the surge resulting from the discharge of lightning. Generally speaking it is desirable that the lightning arrester interrupt the follow current as quickly as possible after the cessation of the discharge current resulting from the lightning, in order not to overload the line or apparatus for which the lightning arrester affords protection, which would in some cases cause severe damage to the equipment, and in others cause the interruption of the power by the operation of circuit breakers and other protective devices. The term duty cycle as employed herein is employed in the same sense as the word is used in a work entitled Lightning Protection for Electrical Systems, by Edward Beck, McGraw-Hill Book Company, Inc., 1954, and also the term is employed herein in accordance with the standards of the American Institute of Electrical Engineers, as published in A.I.E.E. Standards No. 4 for Lightning Arresters.

In lightning arresters of the type to which .this invention relates, where a filler of gas-evolving material is disposed inside a hollow tube of insulating material, which may also be gas-evolving in the presence of a discharge current arc, it is desirable to leave a small space between the inner wall of the tube and the outer diameter of the filler element, be this a purely cylindrical element, or be it a cylindroid element having a spiral groove therein as is the present case. This spacing between the filler and the inner wall of the insulating tube is referred to as diameter clearance, and the extent of this diameter clearance is an important factor in determining surge current capacity and also follow current interruption ability, as measured in the duty cycle test. Generally speaking, where other factors remain the same, the surge current capacity decreases as the diameter clearance decreases, and generally speaking duty cycle performance, that is, the ability to interrupt follow current, improves as the diameter clearance decreases.

It will be readily understood that the discharge of a lightning surge through the arrester sets up very large momentary forces inside the arrester. Great axial thrust may suddenly develop on the filler material, which is transferred to the electrode member at one or both ends of the arrester, and furthermore great stresses may be applied to the inner wall of the tube on the inside of the lightning arrester as a result of the gas which is suddenly evolved from the gas-evolving material as a result of the arc. The large quantity of gas evolved is expelled in a blast through a vent or vents, blowing out the arc gases and de-ionizing the arc path to extinguish the are, thus interrupting the power current which tends to flow through the arrester to ground following the discharge of a lightning surge. This gas pressure builds up so rapidly that even though one or both ends of the arrester are vented to permit the escape of these gases, the sudden formation of the gases results in great pressures applied to the interior wall of the arrester tube. It follows naturally that the surge current capacity can be increased by increasing the diameter clearance, since the increased clearance provides additional room for the current discharge, and also provides additional room or a greater path for the expansion of gases formed by the discharge. Furthermore, a larger portion of the discharge passes down along the wall of the inner tube, that is, the inside wall, and a smaller portion of the discharge passes down the spiraled path of the filler member, so that less axial stress is generated at the moment of discharge.

To increase the discharge current capacity by increasing the diameter clearance, however, would result in a reduction in the follow current interruption capacity of the arrester, since it would be more difiicult to extinguish the arc and de-ionize the arch path, and for other reasons which are well known in the art, and which are set forth in the aforementioned work by Edward Beck.

The arrester of the instant invention overcomes the limitations of the prior art by providing greatly increased mechanical strength for the arrester walls, and in which mechanical stresses during surge current discharge and follow current flow are absorbed by the material of the arrester.

In summary, the arrester of the instant invention avoids the concentration of stresses in the tube material by utilizing a triple thread system which forces the external threads of an inner fiber tube to take part of the axial load. With the tube material near the outside diameter of the inner fiber tube taking up more of the mechanical stresses of the arrester operation, the tube material is better utilized; the arrester has a higher surge current capacity which is not obtained by sacrificing the follow current interruption ability of the arrester.

Accordingly, a primary object of the invention is to provide a new and improved lightning arrester.

Another object is to provide a new and improved lightning arrester characterized by increased surge current capacity.

A further object is to provide a new and improved lightning arrester characterized by high follow current interruption ability.

A still further object is to provide a new and improved lightning arrester in which a triple thread system of mounting an electrode in the insulating tube of the arrester and assembling the arrester parts permits greater utilization of the tube material to oppose stresses created in arrester operation.

These and other objects will become more fully apparent after a study of the following specification, when read in connection with the accompanying drawing, in which the single figure thereof is a side elevation view, partly cut away to show a section, of an arrester according to the invention.

Referring now to the drawing for a more detailed understanding of the invention, there is shown at 11 a cylindrical hollow tube composed of insulating material and preferably composed of some materialsuch as fiber which will emit a substantially un-ionized gas in the presence of an electrical arc. The tube 11 has the inside bore 12 of a precise diameter centrally disposed therein, for reasons which will become hereinafter more clearly apparent. The upper end of the tube 11 is threaded on the outside thereof at and has a tapered portion 9 with threads 8 on the inner surface thereof.

The tube 11 has disposed in the bore therein a spiral diffuser element or filler member 13, which it is understood is composed of a gas-evolving material such as fiber, which has spiral groove 14 extending along the length thereof, and which'has an outer diameter to provide a predetermined spacing 15 between the inner wall 12 and the filler member 13. This spacing may be in the order of 10 mils. The upper end of the spiral filler member 13 is tapered as shown to fit int-o a cup 16 which is machined in the shank end 17 of the electrode 'stem generally designated 18 having a threaded upper portion 19, the threads of which engage the inner threads in the bore 42 of a tapered electrode member generally designated 20,'which has the outer wall thereof tapered outwardly and threaded at 21 to engage the aforementioned threads 8 on the inner wall of the tapered upper portion 9 of the aforementioned tube or sleeve 11. The threaded upper end 19 of the electrode stem has a slot or keyway 40 therein for turning the stem and adjusting its position. The electrode member 20 is composed of any suitable metal or conductive material. It is seen that the electrode member 20 has an upper portion 22 of reduced diameter, this portion of small diameter 22 being adapted to receive a ring-like electrode retainer 23 having the outer wall thereof threaded at 24 to engage some of the threads 29 on the inner wall of an outer fiber tube or sleeve generally designated 25. The ring retaining member 23 has a series of holes 26 spaced around the periphery thereof, for the use of the tool to screw the retaining ring into the assembly. A portion of the threads 29 on the inner wall of the sleeve are in threaded engagement with aforementioned threads 10 on the outer wall of the upper portion of tube 11. A suitable sealing material, such as one known in the trade as. Vistanex may be introduced between threads 10 and threads 29. It is seen that the lower portion 27 of the outer fiber tube 25 has a somewhat larger inner diameter than the upper portion thereof, and disposed between the inner Wall of the lower portion 27 and the outer adjacent wall of the fiber tube 11 is a jacket of steel or other suitable material generally designated 28, which jacket extends along substantially the lower half of the arrester but stops well below member 20 to provide a long jump distance. The steel jacket 28 has disposed in the lower end thereof the lower electrode member 30, which has a plurality of longitudinally-extending bores at spaced intervals around the periphery thereof, one of these bores or vents being shown at 31, the bores being provided for venting the lower interior 32 of the tube 11 and exhausting the gases created by the discharge current, member 30 being composed of any suitable metal. The member 30 is retained in position by crimping the steel jacket 28 in an annular groove 33 in the member 30. Member 30 also has a central bore 34 and it is seen that the member 30 has threads 35 on the exterior thereof for mounting the arrester in a suitable socket, not shown; member 30 also has the flange portion 36 adapted to engage the socket and firmly support the arrester therein.

The lightning arrester shown in the figure and described in connection therewith is well adapted to accomplish the aforementioned objects of the invention. The space 15 between the outer diameter of the spiral filler member 13 and the inside wall 12 of the fiber tube 11 is such that the follow current interruption capabilities of the arrester are not reduced, although the mechanical strength and ability of the arrester tube to withstand the sudden forces created inside the arrester as a result of surge currents are increased by the triple thread system which utilizes the properties of the materials to full advantage. The spacing 15 is maintained small enough to enhance the follow current interruption ability of the arrester. As previously stated, the surge current capacity of an arrestercan be increased by increasing this spacing :but only at the expense of the follow current interruption capabilities, resulting in a less desirable duty cycle performance.

Assume by way of description that the lightning arrester of the figure is installed in a transmission line, for example, and that a high voltage surge is impressed "across the lightning arrester resulting from a lightning discharge. The are which occurs inside the tube 11 between the electrode stem 17 and the electrode 30, even thoughthe electrode 30 is vented by a number of openings 31 and 34, creates a large and sudden pressure inside the tube 11. The force of this gas inside the chamber 7 forces the metallic member 20 in an upward direction as seen in the figure. In addition, movement of gases in groove 14 causes axial thrust of spiral diffuser or spiral filler member 13, which axial thrust is transmitted to stem 17 and thence is transmitted to member 20. The threads of the member 20 more tightly engage the threads 21 on the upper tapered end of the fiber tube 11. It is seen that the threads 8 on the member 20* also taper, the outer diameter becoming somewhat greater at the upper end portion of member 20, as seen in the figure. The engagement of the adjacent threads results in a distribution of force over the threaded area rather than at one point or locality on the wall adjacent member 20. Furthermore, as previously stated, the outer threads 10 of the upper portion of the tube 11 engage the adjacent threads 29 in the outer fiber sleeve 25 and force is transmitted through the upper portion of the .tapered wall portion 9 to the threads between this portion and the outer fiber sleeve 25. Again there is a distribution of forces over a large area because of the thread arrangement, and the result is that stress is not localized at anyone point which might result in a rupture of the wall of one or both of the fiber members 11 and 25'. Furthermore, it is seen that the retaining ring member 23 is threaded at 24, the threads engaging adjacent threads in the outer fiber sleeve 25. Force acting in an upward direction from the chamber 7 upon the member 20 is transmitted to the retaining member 23, and these forces are ultimately applied to the threads 24 and distributed over a large area. The threads then may be thought of as a shock absorbing device for taking large stresses created at the moment of a current discharge and spreading these stresses over large areas of the wall structure. By this utilization of the properties of the material, and the spreading of forces over a large area rather than permitting forces to be concentrated at one or two points in the apparatus, the possibility of rupture from the great forces created at the moment of current discharge is reduced, and this is accomplished without the necessity for increasing the spacing between the filler member 13 and the inner wall of the fiber tube 11.

The adjustable electrode stem 17 assists in preventing mechanical damage to the spiral filler member, and the cup 16 in the end of the stem minimizes the possibility of splitting the tiller member by excessive pressure exerted against the electrode stem.

In tests of the arrester of the invention conducted according to test procedures outlined in the aforereferenced work by Beck entitled Lightning Protection for Electric Systems, and in accordance with the lightning arrester standards of the American Institute of Electrical Engineers, the arrester of the instant invention has been found to have much longer operating life than an unimproved arrester without the triple thread arrangement.

Furthermore, prior art arresters have sometimes employed a polyethylene jacket or sleeve, whereas the apparatus of the instant invention employs fiber. The polyethylene jacket merely performs the function of shielding the electrode; in the arrester of my invention the jacket or sleeve of fiber performs the additional function of providing strength to the wall portion of the structure, by the use of the aforedescribed thread arrangement.

Again by way of summary, the gain in mechanical strength results from addition of material to the tube walls, and the triple system of threads results in a condition such that when the tapered steel plug or electrode member 20 moves upward under load, that is, as a result of internal pressure, physical contact is made with the steel cap or retaining member 23. For this condition, all three threads are loaded, forcing a favorable distribution of stresses in the fiber tube walls.

Performance tests of my improved arrester and a standard arrester of the prior art conducted for comparison purposes demonstrate conclusively that my arrester has increased surge current capacity, better duty cycle performance, and greater voltage withstand ability.

The invention includes the use of a spark gap in series with the arrester so that the fiber tube will not be continuously subjected to the normal line voltage. A gap structure similar to that shown in Patent No. 2,677,072 may be employed, if desired.

It will be understood that the arrester of the figure may be enclosed in a suitable weather-protective housing, not shown for convenience of illustration, of porcelain or other suitable material. A porcelain housing similar to one of those shown in Patents 2,871,390 and 2,677,072 may be employed.

Whereas fiber has been mentioned as a suitable gasevolving material for the tube and filler, other materials may be employed.

Whereas my invention has been shown and described with respect to an exemplary embodiment thereof which gives satisfactory results, it should be understood that changes may be made and equivalents substituted without departing from the spirit and scope of the invention.

I claim as my invention! 1. A lightning arrester comprising, in combination, tube means composed of insulating material, one end of said tube means having internal threads and external threads, threaded electrode means in said end having the threads thereof in engagement with said internal threads, sleeve means composed of insulating material having an internally threaded Wall portion and being disposed around the tube means over at least a portion of the length thereof including said one end, some of the threads of the sleeve means being in threaded engagement with the external threads of the tube means, a threaded electrode retaining member mounted in said sleeve means adjacent the electrode means, filler means composed of insulating material disposed in said tube means, and other electrode means at the other end of the tube means, at least one of the tube means and filler means being composed of a material which evolves gas in the presence of an electric arc, the gases resulting from an are formed by an electrical discharge in said tube means between the electrode means and the other electrode means supplying an axial thrust to the electrode means which is transmitted to the threaded end portion of the tube means and the threaded portion of the sleeve means.

2. A lightning arrester according to claim 1 wherein said one end of the tube means is additionally characterized as being tapered to provide an inside diameter which increases toward the end of the tube means, and said electrode means is additionally characterized as having a wall diameter which tapers over the length thereof, the tapered inner surface of the tube means being in snug threaded engagement with the tapered wall surface of the electrode means.

3. A lightning arrester according to claim 1 wherein the electrode means is additionally characterized as including a threaded bore therethrough and having a threaded electrode stem in said bore, the position of the stem being adjustable with respect to the filler means.

4. A lightning arrester comprising a tubular structure including a tube composed of insulating material, one end of said tube being threaded internally and externally, threaded force-resisting electrode means mounted in said end of said tube in threaded engagement therein, filler means in said tube having one end thereof adjacent said electrode means, at least one of the filler means and tube being composed of a material which evolves a gas in the presence of an electric are, a threaded sleeve mounted around the outside of said tube in threaded engagement therewith, a threaded electrode retaining member disposed in the end of said sleeve adjacent the electrode means and adapted to have force applied thereto by way of the electrode means when gas is generated in said tube, and other electrode means at the other end of said tube adjacent the other end of the filler means, at least one of said electrode means and said other electrode means being vented to permit the escape of gas to the outer atmosphere.

5. A lightning arrester comprising a tubular structure including a tube composed of insulating material, one end of said tube being threaded internally and externally, first threaded means consisting of a sleeve of insulating material disposed around the outside of said tube at said end, the threads of said first threaded means engaging the threads on the external surface of said tube, second threaded 'means consisting of conductive material disposed inside said tube, the threads on said second threaded means engaging the threads on the inner surface of said tube, said second threaded means forming electrode means, an electrode stem for said electrode means and disposed inside said tube, a filler member composed of insulating material disposed in said tube with one end adjacent said electrode stem, at least one of the tube and filler members being composed of a material capable of evolving gas when exposed to an electric arc, and an electrode at the other end of said tube adjacent the other end of the filler member, at least one of said electrode and electrode means having at least one bore therein to vent the inside of said tube to the outer atmosphere.

6. A lightning arrester according to claim 5 wherein the filler member is additionally characterized as having a spiral groove extending along substantially the entire length thereof.

7. A lightning arrester comprising a tubular structure including a tube of insulating material, one end of said tube being threaded internally and externally, a sleeve disposed around the outside of said end of said tube, said sleeve being threaded internally, the threads on said sleeve engaging the threads on the outer surface of said tube, an electrode member mounted in said tube, said electrode member having the wall surface thereof threaded, said last named threaded surface being in threaded engagement with the threads on the inner wall of said tube, said electrode member having a bore therein for receiving and holding in position an electrode stem, one end of said electrode member being of reduced diameter, a retaining ring disposed around said portion of reduced diameter and having the outer wall surface thereof threaded, the threads on the outer wall surface of the retaining ring engaging the adjacent threads on the inner surface of the sleeve member, a filler member composed of insulating material mounted in said tube, one end of said filler member being adjacent said electrode stem, and an additional electrode at the other end of said tube adjacent the other end of said filler member, at least one of the tube and filler members being composed of a material which evolves a gas in the presence of an electric arc, one of said electrodes being vented to permit gas generated in said tube to escape to the outer atmosphere.

8. A lightning arrester comprising, in combination, a tubular structure including a tube composed of insulating material, one end of said tube being threaded internally and threaded externally, said end of the tube having a wall of tapered thickness whereby the inside diameter of the tube increases toward the end of the tube; a sleeve of insulating material disposed around a portion of said tube, said sleeve having threads on the internal wall there of, said last-named threads engaging the threads on the outer surface of said tube; an electrode member disposed in said end of the tube, said electrode member having the outer wall diameter thereof tapered to correspond to the taper of the inner wall of said tube, said outer wall of the electrode member being threaded, the threads of the outer wall of the electrode member engaging the threads on the inner wall of the tube, said electrode member having a threaded bore internally thereof to receive an adjustable electrode stem, said electrode member having a portion of reduced diameter at the end thereof; a retaining ring disposed around, said portion of reduced diameter, the retaining ring being threaded on the external wall thereof, the threads on the external wall of the retaining ring engaging the adjacent threads on the internal wall of the insulating sleeve; a filler member composed of insulating material disposed in said tube, one end of said filler member being adjacent said electrode stern; and an additional electrode member mounted at the other end of said tube adjacent the other end of the filler member, at least one of the tube and filler member being composed of a material which evolves a gas in the presence of an electric arc, at least one of said electrode member and additional electrode member being vented for the escape of gas from the tube, the generation of forces in said insulating tube as a result of an electric discharge therethrough and an expansion of gases therein forcing the threads between the electrode member and the insulating tube, the threads between the insulating tube and the insulating sleeve, and the threads between the retaining ring and the insulating sleeve into close engagement with each other distributing the forces generated by said gas over large areas of the wall portion of the structure.

, 9. A lightning arrester comprising a tubular structure including a tube of insulating material, one end of said tube being threaded internally and externally, a threaded insulating sleeve disposed external to said end of said tube and having threads on the inside thereof in engagement with the threads on the outside of said tube, an electrode member mounted in said tube in threaded engagement with the threads on the inside wall of said tube, said inside wall of said tube and said electrode member having tapered portions along the lengths thereof, an electrode stem mounted in said electrode member and held in position therein, a filler member composed of insulating ma.- terial disposed in said tube, at least one of the tube and filler member being composed of a material which evolves a gas in the presence of an electric are, one end of said filler member being adjacent said electrode stern, an additional electrode member on the other end of said tube adjacent the other end of the filler member, at least one of said electrode member and said additional electrode member being vented to permit the escape of gases generated in said tube as a result of electric discharges therein, the lower portion of said insulating sleeve being of increased inner diameter, and a metallic jacket disposed around the outside of said tube in the space between the lower portion of said sleeve and the outer wall of said tube, said metallic jacket being in electrical contact with said additional electrode member.

10. A lightning arrester comprising a tubular structure including a tube of insulating material having an internal surface capable of evolving gas when exposed to an electric arc, the inner wall and the outer wall of said tube being threaded at one end thereof; an electrode at the other end of said tube; an electrode member having an electrode stem disposed near said first named end of said tube in threaded engagement with threads on the inner wall of said tube and in electrical contact with said stem; a sleeve of insulating material disposed around said tube and having threads on the inner wall thereof in threaded engagement with the threads on the outer wall of said tube; and a retaining ring disposed in said tubular structure between the end of said sleeve and said electrode member, said retaining ring having threads on the outer wall thereof in engagement with adjacent threads on the inner wall of the sleeve, at least one of said electrode and electrode member being vented to permit the escape of gases generated in said tube to the outer atmosphere, gas generated in said tube as a result of electric discharges therein applying pressure to said electrode member causing firm engagement of the threads between said electrode member and said tube, the threads between said tube and said sleeve, and the threads between said retaining ring and said sleeve, all said threads providing for the distribution of forces over a wide area of the material of said tube, sleeve, and retaining member to increase the mechanical strength of said tubular structure.

11. Apparatus according to claim 10 including in addition a filler member disposed in said tube between the electrode and the electrode member, the filler member being made of an insulating material capable of evolving gas when exposed to an electric arc and having a spiral groove in its surface extending continuously around the filler member from end to end thereof, said groove providing an elongated arc path of greater length than the straight initial arc path between the electrode and the electrode'member and being adapted to cause circumferential movement of gas produced by currents of an electric arc in said initial arc path to cause the gas to flow through the groove and to cause the arc to transfer to said elongated arc path in the groove.

12. A lightning arrester comprising a tubular structure including a tube of insulating material having an internal surface capable of evolving gas when exposed to an electric arc, one end of said tube being threaded on the inside and outside thereof, said end of said tube having a tapered inner diameter, an electrode element disposed in said end in threaded engagement with the threads in said tube, said electrode element having a threaded bore therein, a threaded electrode stern mountedin said bore, threaded sleeve supporting means disposed around the outside of said tube at said end in threaded engagement therewith, a threaded retaining ring disposed in the end of said sleeve in threaded engagement therewith, forces generated by gas evolved in said tube applying pressure through said electrode element to said retaining ring and through said electrode element to said tube and sleeve causing the 9 10 threads between the electrode element and the tube, the member, said electrode being vented to permit the escape threads between the tube and the sleeve, and the threads of gases from said tube to the outer atmosphere. between the sleeve and the retaining ring to be more closely engaged and to distribute the forces generated over a Wide References Cited y the Examine! area, a filler member composed of a material which 5 UNITED STATES PATENTS evolves gas in the presence of an electric arc mounted in said tube, one end of said filler member being in substangckermann gg g tial en agement with said electrode stern and an eleclttman 2 1 3,040,203 6/62 Hager 313231 trode at the other end of said tube, said electrode being in substantial engagement with the other end of the filler GEQRGE WESTBY, Primary Examine!- 

8. A LIGHTNING ARESTER COMPRISING, IN COMBINATIN, A TUBULAR STRUCTURE INCLUDING A TUBE COMPOSED OF INSULATING MATERIAL, ONE END OF SAID TUBE BEING THREADED INTERNALLY AND THREADED EXTERNALLY, SAID END OF THE TUBE HAVING A WALL OF TAPERED THICKNESS WHEREBY THE INSIDE DIAMETER OF THE TUBE INCREASES TOWARD THE END OF THE TUBE; A SLEEVE OF INSULATING MATERIAL DISPOSED AROUND A PORTION OF SAID TUBE, SAID SLEEVE HAVING THREADS ON THE INTERNAL WALL THEREOF, SAID LAST-NAMED THREADS ENGAGIN THE THREADS ON THE OUTER SURFACE OF SAID TUBE; AN ELECTRODE MEMBER DISPOSED IN SAID END OF THE TUBE; AN ELECTRODE MEMBER HAVING THE OUTER WALL DIAMETER THEREOF TAPERED TO CORRESPOND TO THE TAPER OF THE INNER WALL OF SAID TUBE, SAID OUTER WALL OF THE ELECTRODE MEMBER BEING THREADED, THE THREADS OF THE OUTER WALL OF THE ELECTRODE MEMBER ENGAGING THE THREADS ON THE INNER WALL OF THE TUBE, SAID ELECTRODE MEMEBR HAVING A THREADED BORE INTERNALLY THEREOF TO RECEIVE AND ADJUSTABLE ELECTRODE STEM, SAID ELECTRODE MEMBER HAVING A PORTION OF REDUCED DIAMETER AT THE END THEREOF; A RETAINING RING DISPOSED AROUND SAID PORTION OF REDUCED DIAMETER, THE RATAINING RING BEING THREADED ON THE EXTERNAL WALL THEREOF, THE THREADS ON THE EXTERNAL WALL OF THE RETAINING 